Serial/De-serial (SERDES) drivers or transmitters include two major modes: a current mode and a voltage mode. For a full swing transmission, e.g., in the Universal Serial Bus (USB) 1.1 version, the voltage mode is used because it consumes less current (i.e., less power) than the current mode. For example, in one application, 5 milliamps (mA) in the voltage-mode is consumed to provide a 1.0 volts (V) peak swing under 50 ohms (Ω) impedance matching, as compared with 20 mA being consumed in the current mode that provides the same/similar result. The output peak swing, however, follows the voltage of the supply voltage and is not controllable.
For a reduced (e.g., de-emphasis) swing data transmission, e.g., used in the USB 2.0 version, the current mode is usually adopted because it is fast and simple. The current-mode transmitter, however, consumes larger power. For example, in one application, 20 mA, versus 5 mA as in the voltage mode, is consumed to provide the same 1.0 V peak swing. Further, under advanced technology, e.g., 28 nanometers (nm) node, the core supply voltage is lower, e.g., about 0.85 V, which causes the driver to suffer headroom unreliability at some operational conditions.
In some approaches, the de-emphasis mode is achieved by turning on a parallel path to change the driving voltage, but the power consumption also increases, e.g., from 5 mA to 7.7 mA in some approaches, to provide a −3.5 decibel (dB) de-emphasis level from the 1.0 V peak swing.
Like reference symbols in the various drawings indicate like elements.